The present invention relates generally to interferometers for laser radar systems and more particularly to interferometers for use with thermoelectrically cooled photoconductive detectors.
As is known in the art, interferometers are used in laser radar systems for determining the beat frequency between transmitted laser signals and target-reflected return signals to thereby determine such target parameters as range and Doppler speed. Such interferometers are used in both homodyne laser radar systems and heterodyne laser radar systems. In a typical interferometer, a laser produces a linearly polarized (such as p-polarized) single (such as TEM.sub.00) mode beam of electromagnetic energy which is directed through a polarization discriminator, such as a Brewster plate, which couples the p-polarized beam to a quarter-waveplate. The quarter-waveplate transforms the polarization of the beam to circular (such as right-circular) polarization. The circularly polarized laser beam is transmitted toward a target, a portion of the transmitted beam being reflected by the target and returned to the interferometer as an oppositely-circularly (such as left-circularly) polarized beam. The quarter-waveplate transforms the polarization of the return beam to linear polarization orthogonal to the linear polarization of the beam produced by the laser (e.g., to an s-polarized beam). The s-polarized beam is focused by a lens, typically a positive meniscus lens, onto substantially a point on a detecting surface of a detector element. In a homodyne interferometer, a portion of the p-polarized beam produced by the laser is deflected and the polarization thereof rotated (such as by a half-waveplate) to a polarization identical to the s-polarized target reflected return beam focused onto the detector, thereby providing an s-polarized local oscillator (L.O.) beam. A separate laser generates the s-polarized L.O. beam in a heterodyne interferometer. The local oscillator beam conventionally is focused by the meniscus lens onto the same point on the detecting surface of the detector element as the target-reflected return beam. That is, substantially a point on the detecting surface is illuminated by superimposed target-reflected return and L.O. beams. The superimposed target-reflected return and L.O. beams are of identical linear polarization and have the same plane wavefronts. The local oscillator beam also has a Gaussian intensity distribution on the detector element, which is derived from the single mode TEM.sub.00 output of the laser. The superimposed signals interfere on the detector element, with the detector element thereby producing a beat frequency signal having a frequency representative of the range of the target, which may be further processed to determine target Doppler speed.
Typically, interferometers for laser radar systems have utilized photovoltaic detectors cooled to approximately 77.degree. K. with liquid nitrogen in order to obtain maximum sensitivity to the incident L.O. beam and target-reflected return beam and to provide the photovoltaic detector with minimum noise equivalent power (NEP). However, photoconductive detectors have recently been developed which need be cooled to only about 190.degree. K, and thus may be cooled by thermoelectric stacks, thereby eliminating the requirement for providing liquid nitrogen to cool the detector element. While conventional interferometers (such as the interferometer discussed above), which focus both the L.O. beam and the target-reflected return beam to substantially a point on the detecting surface of a photoconductive detector, are satisfactory in some applications, such thermoelectrically-cooled photoconductive detector has an NEP approximately 10 dB higher than that provided by a photovoltaic detector in such an interferometer. Thus, the signal-to-noise ratio of a conventional interferometer is decreased by approximately 10 dB when a photoconductive detector is substituted for a photovoltaic detector. Such a large increase in NEP and decrease in signal-to-noise ratio is difficult to compensate for and may not be acceptable in some applications.